docs: def _compute_f1_factor(self, rrup):
          """
          Compute and return factor f1 - see equation (5), 4th term, p. 2191
          """
          # f1 = min(log10(rrup),log10(R1))
          f1 = np.log10(rrup)
          logR1 = np.log10(self.COEFFS_IMT_INDEPENDENT['R1'])
          f1[f1 > logR1] = logR1
  
          return f1